Bragg gratings are structures with a periodic variation in the refractive index that are usually formed in optical components such as holograms, waveguides, and optical fibers. These structures reflect a narrow spectral and angular bandwidth of light that is determined by the average refractive index of the grating, and the spatial period of the refractive index variation.
The fraction of incident light that is reflected Bragg gratings is determined by the magnitude of the refractive index variation and by the number of refractive index periods included in the structure. Reflectivity greater than 99% can be obtained in Bragg structures that are only 100 μm to 300 μm thickness or optical path length and have refractive index changes near 0.002. Accordingly, the spectral bandwidth required by a particular application is accounted for the appropriate choice of hologram thickness, or in the case of waveguides by the optical path length. The spectral bandwidth of the reflected light decreases as the number of refractive index periods increases. Accordingly the spectral bandwidth required by a particular application is easily accounted for by appropriate choice of hologram thickness. A 300 μm thick Bragg grating, for example, with a reflectivity at 500 nm that is greater than 99%, will have a spectral bandwidth that is less than 0.4 nm, full width half maximum (FWHM). This combination of high reflectivity over narrow spectral bandwidth has several interesting applications. Bragg reflection gratings, for example, are used in optical communication as stabilizers for pump lasers, narrowband wavelength division multiplexing (WDM) add/drop filters, and gain-flattening filters. Additional applications include narrowband filters for laser protection, Raman spectroscopy, wireless optical communication, and light detection and ranging systems (LIDAR). In these applications a signal is carried by light of a specific wavelength. It is, therefore, often necessary in such devices to improve their signal-to-noise ratio (SNR) by isolating the signal beam wavelength from polychromatic background light.
Typical uses of airborne LIDAR systems include the detection of submarines and mines, environmental monitoring, and bottom mapping. The signal to noise of LIDAR systems is inversely proportional to the line width of the filter employed, and is directly proportional to the level of detection of the desired wavelength.
Two related problems can limit the direct use of a single, narrow-spectral-bandwidth, reflection hologram to select the desired signal beam. First, reflection of light outside the desired spectral bandwidth can be achieved by changing its incident angle to match the desired Bragg condition of the hologram. However, if this off-wavelength reflected light is allowed to reach the detector of any device employing a holographic filter, the SNR will be reduced. In addition, signal light that is incident outside of a relatively narrow band of incident angles will not be reflected and the detected signal strength will be less than the total signal striking the filter. Further, increasing the thickness of a reflection hologram, or, for example, the optical pathlength of a waveguide, narrows the spectral bandwidth but also reduces the angular field of view.